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<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 400PX;
        }
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<body>
    <div>
        <ul>
            <li>测试</li>
            <li>消失</li>
            <li>边界线</li>
            <li>右侧</li>
            <li>边界线</li>
            <li>消失</li>
            <li>测试</li>
        </ul>
    </div>
</body>

</html>